Plasma arc cutting and welding are processes in which an electric arc is used to perform work, either cutting or welding, on a work piece. The arc creates a plasma that either cuts or welds the work piece (or filler metal used to weld the work piece).
Power sources typically convert a power input to a necessary or desirable power output tailored for a specific application. In welding applications, power sources typically receive a high voltage alternating current (vac) signal and provide a high current output welding or cutting signal. Often, line power sources (sinusoidal line voltages) are either 115V or 230V (or other voltages where one is double the other). These sources may be either single-phase or three-phase and either 50 or 60 Hz. Welding power sources receive such inputs and produce an approximately 10-40 volt dc or ac high current welding output. Cutting power sources produce an output at approximately 90-125 volts.
Generally, a welding or cutting power source is designed for a specific power input. In other words, the power source cannot provide essentially the same output over the multiple input voltages. Further, components which operate safely at a particular input power level are often damaged when operating at an alternative input power level. Therefore, power sources in the prior art have provided for these various inputs by employing circuits which can be manually adjusted to accommodate a variety of inputs. These circuits generally may be adjusted by changing the transformer turns ratio, arranging primary windings in parallel or series, changing the impedance of particular circuits in the power source or arranging tank circuits to be in series or in parallel. However, an improperly linked power source could result in power source failure.
Some prior art welding sources provided an automatic linkage. For example, the Miller Electric AutoLink.TM. is one such power source and is described in U.S. Pat. No. 5,319,533 incorporated herein by reference. Such power sources test the input source voltage when they are first connected and automatically set the proper linkage for the input source voltage that it senses. Such welding power sources, if portable, are generally inverter-type power sources. The inverters may be connected in parallel (for 230V) or in series (for 460V). Such arrangements generally allow for two voltage connection possibilities. This design works well, but it may be costly to implement.
Certain types of power sources (such as inverters or those with torroidal transformers) need to have special control during the initial application of input power in order to control the input current for a period of time (typically 1 to 3 seconds) so that adverse effects are not created. Such adverse effects may include circuit breaker tripping or internal component overload. The circuit that controls primary current during the start up of a power source is often called a precharge circuit. Precharge circuits are found in inverter type power sources as well as conventional type power sources (non-inverter). A typical precharge circuit includes a resistor for limiting current.
Plasma cutting arc power supplies are often designed to be relatively easy to switch back and forth between two input voltages (115 and 230 volts ac, e.g.). Unfortunately, users may accidently plug the unit into the higher ac line voltage when the linking is set for the lower voltage. This doubles the input voltages to the power source, and causes excessive inrush current. If the power supply includes a precharge circuit, a resistor (e.g. 10 ohms) will receive excessive inrush current. This inrush current is often so great that it causes the resistor to melt down, thereby electrically opening the resistor as if it were a fuse. This disables the plasma power supply until the resistor is replaced. This often protects circuitry in the power supply, but necessitates the replacement of the precharge resistor which may be inconvenient and/or expensive. Also, if the power supply is erroneously plugged into a line voltage greater than the highest voltage for which the power supply is designed, then the inrush current would be excessive and likely melt the precharge resistor.
Prior art U.S. Pat. No. 5,627,728, issued to Lubomirsky et al. describes a soft start circuit for trickle charging a capacitor bank. The trickle charge is accomplished using a thermistor in parallel with the power switch. Thus, even when the power switch is off the thermistor charges the capacitors. The thermistors do not protect current from flowing to downstream components, rather they allow current to flow to downstream components even when the switch is opened.
Accordingly, a power supply capable of receiving more than one input and having a precharge circuit that protects components, but does not need replacing in the event of incorrect linking or excessive voltage (i.e. a voltage that will damage components) is desirable.